“Menthyl glutarate” or “l-menthyl glutarate” usually refers to the monomenthyl ester of glutaric acid (monomenthyl glutarate or “MMG”), dimenthyl glutarate (“DMG”), and mixtures thereof. Menthyl glutarate is used as a physiological cooling agent in consumer products such as confectionary and oral care (see, for example, U.S. Pat. Appl. Publ. 2007/0248717). According to Hiserodt et al. (J. Agric. Food Chem. 52 (2004) 3536), both MMG and DMG are found in nature.

Erman et al. disclosed an efficient process for making mixtures of MMG and DMG significantly enriched in the first component (see U.S. Pat. No. 7,247,743). Reaction of menthol with glutaric anhydride in the presence of alkali metal salt catalysts affords mixtures with MMG/DMG ratios ranging from 10 to 20, while a non-catalyzed process produces ratios from 5 to 7.
Hilditch reported a method of preparing pure DMG (J. Chem. Soc. (1909) 1578). This method consists of converting glutaric acid into its chloroanhydride, followed by a reaction of the chloroanhydride with excess menthol.
Marketable menthyl glutarate typically contains 60-70% of MMG and 30-40% of DMG, with a total of MMG+DMG≧97% (see Dewis, “Molecules of Taste and Sensation” in Chemistry and Technology of Flavours and Fragrances, D. Rowe, ed., CRC Press, Boca Raton (2005) Ch. 9, pp. 199-243). However, an efficient process for obtaining such mixtures has not been described in the literature. Of course, MMG and DMG can be obtained separately by the above-mentioned methods and blended in the desired proportion, but this approach is costly. A more economical way to obtain commercially desirable mixtures of MMG and DMG, preferably one that avoids the need for the relatively expensive glutaric anhydride, is still needed.